In response to the demand for more reliable and higher capacity data storage and retrieval systems, there is considerable activity in the research and development of so-called optical disk recording systems. These systems utilize a highly focused modulated beam of light, such as a laser beam, which is directed onto a recording layer which is capable of absorbing a substantial amount of the light. The heat thusly produced causes the light-absorbing material in the areas struck by the highly focused laser beam to change chemically and/or physically, thus producing a concomitant change in optical properties, e.g., transmissivity or reflectivity, in the affected area. For readout, the contrast between the amount of light transmitted or reflected from the unaffected parts of the absorbing layer and from the marked areas of the layer is measured. Examples of such recording systems are disclosed throughout the literature and in numerous U.S. patents such as U.S. Pat. Nos. 3,314,073 and 3,474,457.
In recording data, a rotating disk having a light-absorptive recording layer is exposed to modulated radiation from a laser source. This radiation is passed through a modulator and appropriate optics, and the highly focused laser beam is directed onto the disk which forms by chemical and/or physical reaction of the light-absorbing layer a series of very small marks along a spiral path within the light-absorptive layer. The frequency of the marks is determined by the modulator inputs. Using laser beams with a focused spot diameter of 1 .mu.m or less, data can be stored at a density of 10.sup.8 bits/cm.sup.2 or higher.
The simplest optical disk medium consists merely of a dimensionally stable solid substrate on which is coated a thin layer of light-absorptive material such as a metal layer. When the lightabsorptive layer is struck by an intense beam of coherent light, such as from a laser source, the light-absorptive material is either vaporized and/or thermally degraded, thereby producing a very small marked area which exhibits different transmissivity or reflectivity than the adjacent unmarked area. Multilayer antireflection structures, such as those disclosed in U.S. Pat. No. 4,305,081 to Spong and U.S. Pat. No. 4,270,132 to Bell, increase the absorption of the laser beam which gives better read/write contrast than with the use of simple single layer media. Therefore, for purposes of obtaining better power efficiency, sensitivity and readout response of the record, it has been preferred to use multilayer antireflective structures.
There are two basic types of multilayer antireflective structures, one of which is basically a bilayer structure and the other a trilayer structure. In bilayer media, the substrate is coated with a very smooth, highly reflective material such as aluminum, on top of which is coated a layer of moderately light-absorptive material which is preferably of a thickness corresponding to about .lambda./4n, where .lambda. is the wavelength of the recording light source and n is the refractive index of the light-absorptive layer. In trilayer media, the substrate is likewise coated with a first layer of very smooth highly reflective material on which is coated a second layer of transparent material. Atop the transparent second layer is coated a thin third layer of strongly light-absorptive material. The combined thickness of the transparent and absorptive layers is preferably adjusted to be about .lambda./4n. In both types of structures, the adjustment of certain layer thicknesses according to the wavelength of light and refractive index of the layer is for the purpose of minimizing the amount of light reflected from the unmarked areas and minimizing the amount of light reflected from the marked areas, thus producing a higher playback signal amplitude. A detailed discussion of the three types of disk construction is given by A. E. Bell in Computer Design. January 1983, pp. 133-146 and the references cited therein. See especially Bell and Spong, IEEE Journal of Quantum Electronics, Vol. QE-14, 1978, pp. 487-495.
It will be realized, of course, that the terms "bilayer" and "trilayer" refer only to the fundamental optical layers and do not exclude the use of ancillary layers such as a dust defocussing layer.
Ancillary layers for optical media are frequently made of polymeric materials, However, the deposition of polymer films on optical recording disks is a particularly difficult task because of the very high uniformity requirements for the surfaces of such media. Depending on the nature of the film, such layers may be applied by laying down an integral film, spin coating or spraying a thin polymer solution, in situ plasma polymerization, sputtering or by free surface casting. Each of these has its appropriate niche in the manufacture of optical recording disks. However, each has its disadvantages as well. For example, plasma polymerization and sputtering are satisfactory only for very thin coatings which do not exceed about 5000.ANG. thickness. On the other hand, applying an already-formed film is not useful for films that are less than about 500 .mu.m thickness because of the difficulty of applying them uniformly. Spin coating is useful for certain applications however radial deviations in the thickness of the coating are a problem. Heretofore free rotational casting has not been a useful coating method for annular disks because of the difficulties (1) in applying the coating uniformly over the entire surface of the annulus, (2) in applying the coating at a rate rapid enough to be suitable for commercial production, and (3) in applying the coating in a single rapid pass in such fashion as to avoid a "seam" at the end of the rotation.
The invention is therefore directed to a novel and inventive method for rotational free casting of film-forming liquids upon annular substrates in a single pass in such manner that the seam is insignificant and the coating time is consistent with commercial production rates.